Charged particle beam systems are used in a variety of applications, including the manufacturing, repair, and inspection of micro-fabricated devices, such as integrated circuits, magnetic recording heads, and photolithography masks. Dual beam systems, such as the DualBeam instruments commercially available from FEI Company, the assignee of the present invention, typically include a scanning electron microscope (SEM) that can provide a high-resolution image with minimal damage to the target, and an ion beam system, such as a focused or shaped beam system (FIB), that can be used to alter substrates and to form images. Such dual beams systems are described, for example, in U.S. Pat. No. 7,161,159 to Hill et al., which is incorporated by reference in its entirety in the present application. In some dual beam systems, the FIB is oriented an angle, such as 52 degrees, from the vertical and an electron beam column is oriented vertically. In other systems, the electron beam column is tilted and the FIB is oriented vertically or also tilted. The stage on which the sample is mounted can typically be tilted, in some systems up to about 60 degrees.
A common application for a dual beam system is analyzing defects and other failures during micro-fabrication to troubleshoot, adjust, and improve micro-fabrication processes. Defect analysis is useful in all aspects of semiconductor production including design verification diagnostics, production diagnostics, as well as other aspects of microcircuit research and development. As device geometries continue to shrink and new materials are introduced, the structural complexity of today's semiconductors grows exponentially. Many of the structures created with these new materials are re-entrant, penetrating back through previous layers. Thus, the defects and structural causes of device failure are often hidden well below the surface.
“Deprocessing” means removing structure to expose underlying structure. Deprocessing is sometime necessary to characterize buried structures. Current deprocessing techniques concentrate on delivering data and access to the structure in a planar fashion—mills are crafted to create surface orthogonal to the device surfaces in order to allow imaging, probing, or other localization techniques. Likewise cleaving the wafer or parallel-lapping deprocessing produces a plane of information/access to the structure. Current imaging and fault isolation techniques (microprobing, scanning—capacitance microscopy, voltage-contrast imaging) access this planar surface to provide either structural/metrological data, or electrical information for further isolating the fault.
Accordingly, defect analysis often requires cross-sectioning and viewing defects on a three-dimensional basis. Better systems capable of performing three dimensional defect analyses are more important than ever. This is because there are more defects that are buried and/or smaller, and in addition, chemical analysis is needed in many cases. Moreover, structural diagnostics solutions for defect characterization and failure analysis need to deliver more reliable results in less time, allowing designers and manufacturers to confidently analyze complex structural failures, understand the material composition, and source of defects, and increase yields.
Additionally, while most regions of interest in prior art integrated circuits are confined to a small volume of the integrated circuit (IC) device in a normally planar region (i.e. a SRAM or NAND flash cell occupies a distinct X and Y location, with a small volume of active are in the Z direction), evolving new technologies require more distinct isolation of the volume-of-interest (VOI) in three dimensions. Identification of a current technology region of interest (ROI) typically involves either a X/Y bit address, a gate X/Y address on the die, or some other essentially X/Y localization data because the active area is confined to the substrate wafer surface. Emerging 3D IC fabrication technologies do not constrain the active area to one plane in the Z direction. Active areas have many levels of active devices. X, Y, and Z coordinate information is necessary.
FIG. 1 shows a method for exposing a cross-section using a dual beam SEM/FIB system as known in the prior art. Typically, to analyze a feature within the sample 102, a focused ion beam (FIB) exposes a cross section, or face 108, perpendicular to the top of the surface 112 of the sample material having the hidden feature to be viewed. Because the SEM beam axis 106 is typically at an acute angle relative to the FIB beam axis 104, a portion of the sample in front of the face is preferably removed so that the SEM beam can have access to image the face. One problem with the prior art method is that a large number of cross-sections must typically be exposed along the length of the trench to form a set of samples of a sufficient size to properly characterize the trench.
For features that are deep relative to the opening that is being made by the FIB, the prior art method suffers from a reduced signal to noise ratio. The situation is analogous to shining a flashlight into a deep hole to try to form an image of the side of the hole. For example, a typical copper interconnect trench is 5-8 nanometers (nm) wide by 120 nanometers deep. Many of the electrons from the SEM remain in the trench and are not scattered back to the detector.
Another drawback, for example in defect analysis applications, is that many cross sections have to be taken along the length of the feature to find a defect. This can be a time consuming process. If the defect lies in between cross sections, then the defect may be missed, or more cross sections have to be taken, increasing the length of time of the verification process.
When an ion beam is used to expose a portion of a 3D IC or three dimensional nanoscale structure for analysis, it can be essential to determine precisely not only the X-Y coordinate of the feature of interest, but also Z coordinate, that is, the depth of the feature below the work piece surface. Prior art techniques are not sufficiently accurate for processing features on the nanoscale.
Fiducials are often used to locate a feature of interest on a sample workpiece. Fiducials made on the sample with the FIB at one orientation do not provide optimal features for imaging and subsequent FIB milling references when viewed at another tilted orientation.